Digital Circuit Boards: Mach 1 GHz

2.6 Step Waves on a Properly Terminated Line

When an ideal transmission line is infinite in length there are no reflections. If the line is finite but terminated in a resistance equal to its characteristic impedance, the line will behave as if it is infinite in length. The ideal step wave action along this line is shown in Figure 2.4. After the switch closes, a wave (1) progresses down the line.

Figure 2.4 The wave associated with a line terminated in its characteristic impedance.

This resistor placement is usually called a parallel termination. The terminology shunt termination is also used (see Section 2.17 for series terminated lines).

When the step wave (1) reaches the end of the line, the voltage and current match the requirements of the load, and all wave action stops. There is no radiation after the wave reaches the termination. This wave process occurs whether the transmission line is a trace over a conducting plane or simply two parallel traces. Energy flows in the field between the conductors and not in the conductors themselves. The traces are guides. The analogy with train tracks is appropriate. A train can only go where there are tracks. The only things that leave the track are passengers, heat, and smoke. Then there is the train wreck, where everything leaves the track.